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Versions: 00 draft-sheffer-acme-star

ACME Working Group                                            Y. Sheffer
Internet-Draft                                                    Intuit
Intended status: Standards Track                                D. Lopez
Expires: April 28, 2017                              O. Gonzalez de Dios
                                                          Telefonica I+D
                                                              T. Fossati
                                                                   Nokia
                                                        October 25, 2016


Use of Short-Term, Automatically-Renewed (STAR) Certificates to address
                            the LURK problem
                    draft-sheffer-acme-star-lurk-00

Abstract

   This memo proposes two mechanisms that work in concert to address the
   LURK problem statement, allowing a third party (e.g., a content
   delivery network) to terminate TLS sessions on behalf of a domain
   name owner (e.g., a content provider).

   The proposed mechanisms are:

   1.  An extension to the ACME protocol to enable the issuance of
       short-term and automatically renewed certificates, and
   2.  A protocol that allows a domain name owner to delegate to a third
       party control over a certificate that bears its own name.

   It should be noted that these are in fact independent building blocks
   that could be used separately to solve completely different problems.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 28, 2017.




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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  A Solution for the HTTPS CDN Use Case . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
   3.  Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Preconditions . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Bootstrap . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Refresh . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Termination . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     4.1.  Restricting CDNs to the Delegation Mechanism  . . . . . .   8
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  A Solution for the HTTPS CDN Use Case

   A content provider, and Domain Name Owner (DNO), has agreements in
   place with one or more Content Delivery Networks (CDN) that are
   contracted to serve its content over HTTPS.  The CDN terminates the
   HTTPS connection at one of its edge cache servers and needs to
   present its clients (browsers, set-top-boxes) a certificate whose
   name matches the authority of the URL that is requested, i.e. that of
   the DNO.  However, many DNOs balk at sharing their long-term private
   keys with another organization and, equally, CDN providers would
   rather not have to handle other parties' long-term secrets.  This
   problem has been discussed at the IETF under the LURK (limited use of
   remote keys) title.

   This document proposes a solution to the above problem that involves
   the use of short-term certificates with a DNO's name on them, and a



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   scheme for handling the naming delegation from the DNO to the CDN.
   The generated short-term credentials are automatically renewed by an
   ACME Certification Authority (CA) [I-D.ietf-acme-acme] and routinely
   rotated by the CDN on its edge cache servers.  The DNO can end the
   delegation at any time by simply instructing the CA to stop the
   automatic renewal and let the certificate expire shortly after.

   Using short-term certificates makes revocation cheap and effective
   [Topalovic] [I-D.iab-web-pki-problems] in case of key compromise or
   of termination of the delegation; seamless certificate issuance and
   renewal enable the level of workflow automation that is expected in
   today's cloud environments.  Also, compared to other keyless-TLS
   solutions [I-D.cairns-tls-session-key-interface]
   [I-D.erb-lurk-rsalg], the proposed approach doesn't suffer from
   scalability issues or increase in connection setup latency, while
   requiring virtually no changes to existing COTS caching software used
   by the CDN.

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   TODO: glossary.

3.  Protocol Flow

   The protocol flow can be split into two: a LURK interface, used by
   CDN and DNO to agree on the name delegation, and the ACME STAR
   interface, used by DNO to obtain the short-term and automatically
   renewed certificate from the CA, which is eventually consumed by the
   CDN.  The latter is also used to terminate the delegation, if so
   needed.

   The following subsections describe the preconditions (Section 3.1),
   and the three main phases of the protocol:

   o  Bootstrap: the CDN requests from the DNO the delegation of a
      specific name and in turn DNO asks an ACME CA to create the
      corresponding short-term and auto-renewed (STAR) certificate
      (Section 3.2);
   o  Auto-renewal: the ACME CA periodically re-issues the short-term
      certificate and posts it to a public URL (Section 3.3);
   o  Termination: the DNO (indirectly) stops name delegation by
      explicitly requesting the ACME CA to discontinue the automatic
      renewal of the certificate (Section 3.4).



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3.1.  Preconditions

   The protocol assumes the following preconditions are met:

   o  A mutually authenticated channel between CDN and DNO pre-exists.
      This is called "LURK channel" and all LURK protocol exchanges
      between CDN and DNO are run over it.  It provides the guarantee
      that the LURK requests and responses are authentic [[_1: Note
      that, under this assumption, the key used to authenticate the CDN
      to the DNO becomes a critical asset for the security of the
      proposed protocol, and that certain interactions (e.g., CSR
      submission) might require a stronger authentication mechanism.
      For example, stacking a further authentication factor on top of
      CDN's LURK key would allow to distinguish an attacker that has
      only managed to successfully attack the CDN's LURK key from the
      legitimate CDN. --tf]].
   o  CDN and DNO have agreed on a "CSR template" to use, including at a
      minimum:

      -  Subject name (e.g., "somesite.DNO.com"),
      -  Validity (e.g., 24 to 72 hours),
      -  Requested algorithms,
      -  Key length,
      -  Key usage.

      The CDN is required to use this template for every CSR created
      under the same delegation.
   o  DNO has registered through the ACME interface exposed by the
      Certificate Authority (CA) using the usual ACME registration
      procedure.  The DNO shall, at the registration stage, query the
      ACME server for the supported STAR capabilities - for example: the
      minimum validity period of the issued certificate, the maximum
      duration of the automatic renewal process (either as a maximum
      number of renewal events, or as its maximum absolute life-span).

3.2.  Bootstrap

   CDN (LURK client) generates a key-pair, wraps it into a Certificate
   Signing Request (CSR) according to the agreed CSR template, and sends
   it to the DNO (LURK server) over the pre-established LURK channel.
   The DNO uses the CDN identity provided on the LURK channel to look up
   the CSR template that applies to the requesting CDN and decides
   whether or not to accept the request.  (TBD: This is probably a case
   that would require a further authentication stage over the one
   provided by the mutual-authenticated LURK channel?)  Assuming
   everything is in order, it then "forwards" the CDN request to the
   ACME CA by means of the usual ACME application procedure.




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   Specifically, DNO, in its role as a ACME/STAR client, requests the CA
   a STAR certificate, i.e., one that:

   o  Has a short validity (e.g., 24 to 72 hours);
   o  Is automatically renewed by the CA for a certain period of time;
   o  Is downloadable from a (highly available) public link without
      requiring any special authorisation.

   Other than that, the ACME protocol flows as normal between DNO and
   CA, in particular DNO is responsible for satisfying the requested
   ACME challenges until the CA is willing to issue the requested
   certificate.  The DNO is given back a unique identifier for the
   issued STAR certificate to be used in subsequent interaction with the
   CA (e.g., if the certificate needs to be terminated.)

   Concurrently, a 202 response has been sent back to the CDN with an
   endpoint to poll for completion of the certificate generation
   process.

   The bootstrap phase ends when the DNO obtains the OK from the ACME CA
   and posts the certificate's URL to the "completion endpoint" where
   the CDN can retrieve it.  The information that is passed on to the
   CDN at this stage also includes details about how much time before
   the certificate expires can the CDN expect the replacement to be
   ready.


























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                      ...........................
 LURK                 :  LURK          ACME/STAR:              ACME/STAR
 Client               :  Server         Client  :               Server
   |                  :    |               |    :                  |
   |                  :    |               |   ACME registration   |
   +-------.          :    |               |<--------------------->|
   |       |          :    |               |   STAR capabilities   |
   |   generate CSR   :    |               |    :                  |
   |       |          :    |               |    :                  |
   |<------'          :    |               |    :                  |
   |                  :    |               |    :                  |
   |     Request new  :    |               |    :                  |
   +---------------------->|               |    :                  |
   |     cert for CSR :    |               |    :                  |
   |                  :    +-------.       |    :                  |
   |                  :    |       |       |    :                  |
   |                  :    |   Verify CSR  |    :                  |
   |                  :    |       |       |    :                  |
   |                  :    +<------'       |    :                  |
   |   Accepted, poll at   |               |    :                  |
   |<----------------------+               |    :                  |
   |    "completion URL"   |- - - - - - - >|    Application for    |
   |                  :    |               +---------------------->|
   |                  :    |               |    STAR certificate   |
   |                  :    |               |    :                  |
   |  GET "completion URL" |               |    :  Challenge       |
   |<--------------------->|               |<--------------------->|
   |   202, in progress    |               |    :  Response        |
   |                  :    |               |    :                  |
   |                  :    |               |  Finalize/Certificate |
   |                  :    |               |<----------------------+
   |  GET "completion URL" |< - - - - - - -|    : + STAR Id        |
   +---------------------->|               |    :                  |
   |                  :    |               |    :                  |
   |  200, certificate URL |               |    :                  |
   |<----------------------+               |    :                  |
   |   and other metadata  |               |    :                  |
   |                  :    |               |    :                  |
                      `.........................'

                            Figure 1: Bootstrap

3.3.  Refresh

   The CA automatically re-issues the certificate (using the same CSR)
   before it expires and publishes it to the URL that the CDN has come
   to know at the end of the bootstrap phase.  The CDN downloads and
   installs it.  This process goes on until either:



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   o  DNO terminates the delegation, or
   o  Automatic renewal expires.

           LURK                    ACME/STAR
           Client                  Server
             |     Retrieve cert     |                     [...]
             |<--------------------->|                      |
             |                       +------.              /
             |                       |      |             /
             |                       | Automatic renewal :
             |                       |      |             \
             |                       |<-----'              \
             |     Retrieve cert     |                      |
             |<--------------------->|                   72 hours
             |                       |                      |
             |                       +------.              /
             |                       |      |             /
             |                       | Automatic renewal :
             |                       |      |             \
             |                       |<-----'              \
             |     Retrieve cert     |                      |
             |<--------------------->|                   72 hours
             |                       |                      |
             |                       +------.              /
             |                       |      |             /
             |                       | Automatic renewal :
             |                       |      |             \
             |                       |<-----'              \
             |                       |                      |
             |         [...]         |                    [...]

                          Figure 2: Auto renewal

3.4.  Termination

   DNO requests termination of the STAR certificate by including the
   previously obtained identifier in a STAR certificate termination
   request to the ACME interface.  After CA receives and verifies the
   request, it shall:

   o  Cancel the automatic renewal process for the LURK certificate;
   o  Change the certificate publication resource to return an error
      indicating the termination of the delegation to external clients,
      including the CDN;

   Note that it is not necessary to explicitly revoke the short-term
   certificate.




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   LURK                   ACME/STAR               ACME/STAR
   Client                  Client                  Server
     |                       |                       |
     |                       |  Terminate STAR Id    |
     |                       +---------------------->|
     |                       |                       +-------.
     |                       |                       |       |
     |                       |                       |  End auto renewal
     |                       |                       |  Remove cert link
     |                       |                       |  etc.
     |                       |                       |       |
     |                       |         Done          |<------'
     |                       |<----------------------+
     |                       |                       |
     |                                               |
     |                 Retrieve cert                 |
     +---------------------------------------------->|
     |                 Error: terminated             |
     |<----------------------------------------------+
     |                                               |

                           Figure 3: Termination

4.  Security Considerations

   o  CDN's client certificate key is first order security asset and
      MUST be protected.  Absent 2FA/MFA, an attacker that can
      compromise the key might be able to obtain certificates bearing
      DNO's identity.
   o  Consider collusion of two or more CDNs with contracts with the
      same DNO (?)

4.1.  Restricting CDNs to the Delegation Mechanism

   Currently there are no standard methods for the DNO to ensure that
   the CDN cannot issue a certificate through mechanisms other than the
   one described here, for the URLs under the CDN's control.  For
   example, regardless of the STAR solution, a rogue CDN employee can
   use the ACME protocol (or proprietary mechanisms used by various CAs)
   to create a fake certificate for the DNO's content.

   The best solution currently being worked on would consist of several
   related configuration steps:

   o  Make sure that the CDN cannot modify the DNS records for the
      domain.  Typically this would mean that the content owner
      establishes a CNAME resource record from a subdomain into a CDN-
      managed domain.



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   o  Restrict certificate issuance for the domain to specific CAs that
      comply with ACME.  This assumes universal deployment of CAA
      [RFC6844] by CAs, which is not the case yet.
   o  Deploy ACME-specific methods to restrict issuance to a specific
      authorization key which is controlled by the content owner
      [I-D.landau-acme-caa], and/or to specific ACME authorization
      methods.

   This solution is recommended in general, even if an alternative to
   the mechanism described here is used.

5.  Acknowledgments

   This work is partially supported by the European Commission under
   Horizon 2020 grant agreement no. 688421 Measurement and Architecture
   for a Middleboxed Internet (MAMI).  This support does not imply
   endorsement.

6.  References

6.1.  Normative References

   [I-D.ietf-acme-acme]
              Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
              Certificate Management Environment (ACME)", draft-ietf-
              acme-acme-03 (work in progress), July 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

6.2.  Informative References

   [I-D.cairns-tls-session-key-interface]
              Cairns, K., Mattsson, J., Skog, R., and D. Migault,
              "Session Key Interface (SKI) for TLS and DTLS", draft-
              cairns-tls-session-key-interface-01 (work in progress),
              October 2015.

   [I-D.erb-lurk-rsalg]
              Erb, S. and R. Salz, "A PFS-preserving protocol for LURK",
              draft-erb-lurk-rsalg-01 (work in progress), May 2016.








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   [I-D.iab-web-pki-problems]
              Housley, R. and K. O'Donoghue, "Problems with the Public
              Key Infrastructure (PKI) for the World Wide Web", draft-
              iab-web-pki-problems-03 (work in progress), September
              2016.

   [I-D.landau-acme-caa]
              Landau, H., "CA Account URI Binding for CAA Records",
              draft-landau-acme-caa-01 (work in progress), October 2016.

   [RFC6844]  Hallam-Baker, P. and R. Stradling, "DNS Certification
              Authority Authorization (CAA) Resource Record", RFC 6844,
              DOI 10.17487/RFC6844, January 2013,
              <http://www.rfc-editor.org/info/rfc6844>.

   [Topalovic]
              Topalovic, E., Saeta, B., Huang, L., Jackson, C., and D.
              Boneh, "Towards Short-Lived Certificates", 2012,
              <http://www.w2spconf.com/2012/papers/w2sp12-final9.pdf>.

Authors' Addresses

   Yaron Sheffer
   Intuit

   EMail: yaronf.ietf@gmail.com


   Diego Lopez
   Telefonica I+D

   EMail: diego@telefonica.es


   Oscar Gonzalez de Dios
   Telefonica I+D

   EMail: oscar.gonzalezdedios@telefonica.com


   Thomas Fossati
   Nokia

   EMail: thomas.fossati@nokia.com







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